The production of alcohols from carbon oxides and hydrogen is well-known in the art. For example, a number of processes are known which use catalysts which are known to catalyse the reaction, including those based on Group VI metals, especially molybdenum, as described, for example in U.S. Pat. No. 4,752,623 and U.S. Pat. No. 4,831,060, and those based on mixed metal oxides, especially based on copper and cobalt containing catalysts, as described, for example, in U.S. Pat. No. 4,122,110 and U.S. Pat. No. 4,780,481. More recent publications include WO 2007/003909 A1, which also describes a process for the conversion of carbon oxide(s) and hydrogen containing feedstocks into alcohols in the presence of a particulate catalyst.
The catalytic routes generally produce a mixed alcohols product slate, including methanol, ethanol and heavier alcohols, especially propanol and butanols. The selectivity to the various alcohol products depends on the particular catalyst and process conditions employed and, although both methanol and the higher alcohols (ethanol and above) are usually formed in any particular reaction, the art generally seeks to maximise either methanol or the higher alcohols at the expense of the other.
There are also known processes for the conversion of carbon monoxide and hydrogen into C2+ alcohols based on fermentation processes using bacteria. Examples of fermentation processes can be found, for example, in WO 02/08438 and WO 00/68407, and are also described in DOE reports under DOE Contract Number DE-AC22-92PC92118, such as “Bench-scale Demonstration of Biological Production of Ethanol from Coal Synthesis Gas”, Topical Report 5, November 1995.
In general such processes are much more selective for specific alcohols, such as ethanol, compared to catalytic processes, with much lower quantities, if any, of other alcohols being formed.
The carbon monoxide and hydrogen for such processes can be obtained by reforming of methane-containing feedstocks, such as natural gas, to produce a mixture of carbon monoxide, hydrogen and carbon dioxide (synthesis gas). A number of methane reforming processes and variants thereon are known in the art, principally:
(1) steam methane reforming (SMR), in which the methane containing feedstock is reformed in an externally fired reformer in the presence of >2:1 molar steam:methane ratio (usually >2.5:1),
(2) autothermal reforming (ATR), in which the methane containing feedstock is reformed in the presence of steam and oxygen, and
(3) partial oxidation (PDX), in which the methane containing feedstock is reformed in the presence of oxygen and relatively low or zero concentrations of steam
Significant variations on the above 3 processes are also known, and thus, for example, carbon dioxide can be added to steam methane reforming or autothermal reforming to adjust the ratio of hydrogen to carbon monoxide obtained. In a particular example, dry gas reforming is a variation of steam methane reforming in which the methane containing feedstock is reformed in the presence of significant concentrations of carbon dioxide and low or zero concentration of feed steam—the feed CO2 has the effect of reducing the H2:CO ratio and the low water content allows more effective conversion of CO2 to CO.
In general, however, the ratio of hydrogen to carbon monoxide obtained is decreased in the order (1)>(2)>(3), with a typical SMR reformer (1) having an H2:CO molar ratio of approximately 4.5:1 versus 2:1 for an ATR reformer (2) and 1.7 or 1.8:1 for a PDX reformer (3). (Unless stated otherwise, all ratios herein are molar ratios)
Each of the above processes also produces carbon dioxide. As well as the highest carbon monoxide to hydrogen ratios, ATR and PDX also result in the lowest carbon dioxide and methane in the resulting synthesis gas. Typically an SMR produces syngas with a molar ratio of CO2:CO in the region of 0.35:1 versus 0.2:1 for an ATR and <0.1:1 for a PDX.
In theory, both catalytic and fermentation routes to higher alcohols (ethanol and heavier alcohols) may utilise CO2 as a reactant for the production of the higher alcohols. However, in practise, both catalytic and fermentation routes to higher alcohols tend to be net producers of carbon dioxide.
In the case of catalytic conversions, such reactions may utilise the carbon dioxide via “direct” conversion or via co-occurrence of the water-gas shift reaction, CO2+H2CO+H2O. However, whilst for methanol production, the production can occur directly from CO2, most higher alcohol catalysts appear only to be able to react CO2 via the shift reaction, and at the typical higher alcohol catalyst operating conditions of 250-400° C., the shift equilibrium favours CO2 over CO—and results in the net production of CO2 over the catalyst.
In the case of fermentation routes, the bacteria used for fermentation can produce alcohols according to either of the following 2 reactions6CO+3H2C2H5OH+4CO2 2CO2+6H2C2H5OH+3H2O
However, the CO conversion is typically 70-90% per pass while the H2 conversion is typically less than the CO conversion—therefore the fermentation is also a net producer of CO2.
Based on the above, it may be expected that where fermentation is used to produce alcohols from synthesis gas, such processes would operate most favourably with synthesis gas with the highest concentrations of carbon monoxide and lowest proportion of CO2 to CO, which would favour ATR and PDX over SMR.